This invention pertains to refractory radiation mask fabrication and more specifically to methods of forming a hard mask having less stress during the fabrication of refractory radiation masks.
Generally, for convenience and because standard semiconductor tools can be used in the fabrication process, refractory radiation masks are formed on a semiconductor wafer, such as a silicon wafer. The silicon wafer operates like a frame and support for the mask. A thin layer that ultimately becomes the membrane is deposited on the upper surface of the wafer. A layer of radiation absorbing material, such as some safe heavy metal or alloy, is deposited on the upper surface of the membrane layer. The radiation absorbing layer is patterned by applying a hard mask material and resist. The resist is patterned (or exposed) with an electron beam (E-beam) device and a hard mask is formed by etching the hard mask layer through the patterned resist layer. The hard mask is then used as an etch mask to pattern the radiation absorbing layer. At some point in the process the wafer is etched from the membrane layer in a circle or a rectangle to form a thin membrane. The mask thus allows radiation to pass through the thin membrane and portions of the radiation absorbing layer that have been etched away. The entire procedure is known as a process flow and two different process flows are commonly used.
In the first process flow, commonly referred to as a wafer flow, all processing is done on the wafer with one of the final steps being the back etching of the silicon wafer to form the membrane. The wafer flow was primarily created to solve formatting issues. It allows refractory radiation mask processing to be conducted in semiconductor tools that are not dramatically different from the standard wafer processing tools supplied by the industry. The refractory radiation mask specific processing steps (membrane formation and wafer mounting to a support ring) are at the end of the flow. This minimizes the modifications necessary to both the tools and the wafer processes. However, the creation of the membrane and the mounting of the wafer creates significant pattern displacement errors (xcx9c0.15 xcexcm) in the mask.
The second process flow is commonly referred to as a membrane flow. In the membrane flow the membrane is formed early in the process (generally after hard mask deposition) and the remaining processing is carried out on the membrane. The membrane flow process was derived to remove the errors in the wafer flow process by conducting the mask specific processing steps before the absorber layer is patterned. While this greatly reduces the errors associated with membrane formation and wafer mounting, it also increases the modifications to both the equipment (the tools must accept a refractory radiation mask format rather than a wafer) and the processes (the patterning defining process steps are carried out on a membrane rather than a wafer).
The major problem encountered in the formation of a refractory radiation mask is that the membrane is not a rigid structure. Stress or stress gradients present in the layers being patterned will result in distortion of the mask. It has been found that stress gradients have more impact on image placement than does a uniform stress. Conventionally, chromium (Cr) is used as a hard mask material. However, the ability to repeatably and controllably deposit Cr at zero stress has not been demonstrated. Another widely used hard mask material is silicon dioxide (SiO2). SiO2, which for refractory radiation mask applications is typically deposited in a PECVD system, usually suffers from a high compressive stress.
It is a purpose of the present invention to provide new and improved methods of fabricating refractory radiation masks.
It is another purpose of the present invention to provide new and improved methods of fabricating refractory radiation masks which have less distortion during pattern writing so as to greatly improve the accuracy.
It is a further purpose of the present invention to provide new and improved methods of fabricating refractory radiation masks including the use of a new hard mask fabrication method which substantially reduces distortion during fabrication of the refractory radiation mask.
Briefly, to achieve the desired objects of the instant invention in accordance with a preferred embodiment thereof, provided is a method of forming a hard mask for use in the formation of a refractory radiation mask including providing a membrane structure, forming a radiation absorbing layer to be patterned on the membrane structure, forming a hard mask layer on the surface of the membrane structure, the hard mask layer including a material system having reduced stress and therefore reduced distortion of the membrane structure, and patterning the hard mask layer.
In a further embodiment, the step of forming the hard mask includes forming the material system that is a cross between an oxide and a nitride.
Also provided is a method of fabricating a refractory radiation mask including providing a wafer with a planar surface, forming a membrane layer on the planar surface of the wafer, forming a layer of radiation absorbing material on the membrane layer, forming a hard mask on the layer of radiation absorbing material, the hard mask including a material system having nominally zero stress and therefore reduced distortion of the membrane structure, defining a pattern on the layer of radiation absorbing material, and forming the pattern through the layer of radiation absorbing material to form a refractory radiation mask.
In another embodiment, the hard mask includes a material system having a first material under compressive stress and a second material under tensile stress the stresses of the first material and the second material substantially offset to reduce stress in the hard mask layer.